Efficient Organic Light-Emitting Diodes Obtained by Introducing Gadolinium (Gd) Complexes Based on Pyrazolone Derivative Ligands as Hole Trappers.

The utilization of lanthanide (Ln) complexes in the realm of organic light-emitting diodes (OLEDs) has garnered extensive interest, particularly in their role as luminescent materials or electron trappers. A series of gadolinium (Gd) complexes with energy levels of high HOMO/LUMO and different triplet state energies were designed and synthesized by introducing substituents with different electronic effects onto the pyrazolone derivative ligands. Subsequently, these complexes were precisely purified by vacuum sublimation and codoped into the light-emitting layer (EML) of the OLEDs. This process was facilitated through the well-matched HOMO/LUMO levels and triplet energies among various functional materials. Consequently, the maximum external quantum efficiencies of blue, red, and green OLEDs were simultaneously enhanced with the ratios of 119%, 28%, and 71%, respectively. This improvement can be credited to the introduction of Gd(III) complex molecules within EMLs, which helps to capture excess holes and improve carriers' balance.